Trench power MOSFET with planarized gate bus

ABSTRACT

Power MOSFETs and fabrication processes for power MOSFETs use a continuous conductive gate structure within trenches to avoid problems arising from device topology caused when a gate bus extends above a substrate surface. The conductive gate structure forms gates in device trenches in an active device region and forms a gate bus in a gate bus trench. The gate bus trench that connects to the device trenches can be wide to facilitate forming a gate contact to the gate bus, while the device trenches can be narrow to maximize device density. CMP process can be used to planarize the conductive gate structure and/or overlying insulating layers. The processes are compatible with processes forming self-aligned or conventional contacts in the active device region.

BACKGROUND

[0001] The fabrication of trench-gated devices is common to many highperformance power semiconductor devices today. Such devices include thetrench-gated vertical power MOSFET (a.k.a. trench DMOS, trench FET,UMOS, etc.) and its many variants and derivatives, relying on verticalcurrent flow through a semiconductor from a topside metal contact to ametalized backside contact whereby the potential on a trench-embeddedgate electrode controls the current in said device. Such embedded trenchgates actually include the gate electrode of a MOSFET whose channel iselectrically induced on the sidewall of an etched silicon trenchperpendicular to the wafers surface. Devices employing a trench-gatedvertical channel differ dramatically from conventional MOSFETs whosegate is located above and parallel to the wafer's surface (rather thaninside a trench etched into the silicon). The vertical channel allowsmore MOSFET gate width W to be formed in a given area of silicon realestate. The greater channel density in turn achieves a lower resistanceMOSFET than devices having less gate perimeter in the same area. Suchconventional trench-gated power MOSFETs are well known in the art, beingmanufactured since the early 1990's.

[0002] As shown in the fabrication sequence of FIGS. 1A to 1H,manufacturing a trench-gated vertical MOSFET requires a polysilicontrench gate electrode to be formed and embedded in an oxide-lined etchedsilicon trench. One complexity in the fabrication involves etching thetrench, oxidizing it with a high quality oxide and filling the trenchwith a highly doped layer of polysilicon. For example, in FIG. 1A,silicon substrate having N-type epitaxial layer 10 is masked andpatterned by mask layer 11 (e.g., photoresist or an etched oxide hardmask) to expose the silicon surface to etching in mask opening 12. Thesilicon is then etched using a reactive-ion-etch (RIE) to form a trenchtypically of dimensions approximately 1.5 to 2 μm in depth and 0.8 to0.3 μm in width. The trench is then oxidized, and the oxide is removed(to eliminate crystal defects resulting from the etching process). Thetrench is then re-oxidized to form gate oxide 16, with a thicknessbetween 100 Å to 1000 Å and typically around 250 Å to 500 Å. As shown inFIG. 1B, the trench is then filled with polysilicon layer 13 to athickness sufficient to fill the trench in a void-free manner (having anembedded polysilicon portion 17A) and extend out of the trench and abovethe wafer's surface (so that the surface of polysilicon layer 13 becomesrelatively flat).

[0003] Another challenge during wafer fabrication involves the steps ofetching this gate polysilicon down into the trench and sealing it withan insulator so that it doesn't electrically short to a thick layer ofsource metal, said metal being formed subsequent to the trench gate andcovering much of the wafer's surface. In FIG. 1C, this refill processsequence involves masking and etching back the polysilicon layer intothe trench so that only the embedded portion 17A of the polysilicon gateremains in the active device array. After the polysilicon etch back, thepolysilicon gate may be co-planer with the surface but typically isetched down into the trench (by less than about 0.4 μm) leavingdepression 19 atop the trench. Then as shown in Fig 1D, the silicon andpolysilicon surfaces are thermally oxidized to form oxide layer 20(having a thickness of 100 to 400 Å), followed by boron body implant 21Aand 21B into all the active silicon mesa regions, typically having animplant dose of 8E12 cm⁻² to 3E14 cm⁻² at an energy of 80 to 120 keV toform a shallow implanted layer. This layer is subsequently diffused at ahigh temperature of 900° C. to 1150° C. for 3 to 15 hours to form theactive channel region of the device P-type body region P_(B) 20A and20B, as shown in FIG. 1E. The depth of body regions 20A and 20B aretypically 80 to 85% of the depth of the trench. Heavily doped N+ sourceregions 25A and 25B are then implanted typically with a dose of 5E15cm⁻² (or greater) of arsenic at 80 to 120 keV. Glass layer 20, e.g.,comprising BPSG (borophosphosilicate glass), is then deposited andmasked by a contact mask to expose a portion of the N+ silicon mesaregions 25A and 25B as shown in FIG. 1F, followed by metallization with1 to 4 μm thick aluminum, Al—Si, or Al−Cu—Si layer 31.

[0004] Although the metal is subsequently masked and etched to separategate and source metal connections, in the active cell array shown inFIG. 1H, the entire array is metalized by source metal layer 31. Inevery cell, glass capping layer 30 must prevent electrical shortsbetween source metal 31 and all the embedded polysilicon gates 17A, 17B,and 17C. If any one cell out of millions of cells comprising a singledevice becomes shorted, the entire device is ruined.

[0005] To further complicate the structure and its manufacturing, MOSFET50 in FIG. 2A has a polysilicon gate electrode 54A and 54B that extendsbeyond the confines of the trenches and overlap up and onto the siliconsurface, typically upon a thick field oxide region 53. The portion 55 ofthe polysilicon gate extending outside the trench is needed at least inone location in a device to facilitate gate contact between the embeddedportions 54A and 54B of polysilicon gate and a metalized gate bondingpad (not shown). The field oxide 53 helps minimize the gate to sourcecapacitance.

[0006] Outside the trench and atop field oxide 53, the polysiliconregion 55 may extend for significant distances on and along the die'ssurface (i.e., outside the trench) to aid in the propagation of gatesignals, i.e., to “bus” a signal, across a large area power device. Suchpolysilicon gate bus regions may be shorted by the topside metal alongtheir entire length. But, since most vertical power devices employ asingle layer metal process (the metal generally being several micronsthick), such gate bus regions can only be metalized by interrupting thesource metal. Source metal ideally, however, should cover the die'ssurface to its maximum possible extent (for the lowest possibleresistance device). The need for a metalized gate bus thereforeconflicts with the need for uninterrupted top source metal, forcing anundesirable tradeoff between fast switching speeds and the lowestpossible resistance device.

[0007] The requirement for a metalized gate bus originates from the highsheet resistance of the device's embedded polysilicon gate. Even in-situdoped N+ polysilicon exhibits a high sheet resistance of 30 ohms persquare, preventing signals from being bussed over intra-chip dimensionsat high speeds without the assistance of regularly-distributed metalizedgate-bus structures.

[0008] Also, in vertical MOSFET 50 in FIG. 2A, embedded polysilicon 54Bextends out of the trench as polysilicon portion 56 crossing active area57 before stepping up onto field oxide 53. Since the polysilicon crossesover active area 57 and thin gate oxide 62B, deep P+ region 52 must beformed beneath region 57 to prevent the thin oxide from permitting highelectric fields. To form the deep P+ region 52 beneath polysilicon 56 byconventional implant and diffusion methods, the implant must precede thedeposition of the polysilicon layer, preventing deep P+ region fromusing the same implants used to form P-type body regions 60A and 60B.Moreover, deep P+ must also precede field oxide 53 if it is to extendbeneath the field oxide 52.

[0009] A difficulty with manufacturing MOSFET 50 of FIG. 2A is thenon-planar topography present during critical photolithography andplanarization operations. The depression of the etch-back region 66, forexample, is difficult to etch uniformly. The thicker the layer ofpolysilicon and the thicker the field oxide, the larger the step heightspresent in the topography of the wafer during subsequent processing.Extreme topography can create problems during photomasking, duringetching, and during planarization operations, and virtually precludesthe use of chemical mechanical polishing (CMP) techniques. Large stepheights may also lead to step coverage problems for depositions andconformal coatings.

[0010] Other complications in trench MOSFET manufacturing may occur atany number of steps in the fabrication sequence, and may laterexacerbate issues with trench filling, planarization, and topography.For example in FIG. 2B, device 70 includes an array of trenches 73A,73B, and 73C spaced at regular intervals as defined by mask 72A, 72B,72C and 72D. Trench 73C represents the last trench of the regular array,whereas trench 73D represents a more distant trench, either as part of agate bus region or a termination region distinct from the repeatedarray. During processing, the last trench in the array and the isolatedtrench regions 73C and 73D may etch differently from the regular array.Non-symmetric features 75 in trench 73C and feature 76 in trench 73D mayresult from optical effects during photolithography and also frommicro-loading effects during etching. The misshaped trenches become verydifficult to planarize using polysilicon overfill and etch-back methodssince the trench opening is wider than the other trenches within thedevice.

[0011] Even without misshaped trenches planarization using etch-back canbe challenging. FIG. 2C illustrates trench 81 (etched into epitaxiallayer 80), lined with gate oxide 82 and filled with polysilicon 83.Deposited oxide 84 is then deposited and etched back to protect theembedded gate from shorting to the top metal. After deposition thesurface is relatively planar compared to the thickness of the depositedglass 84. After etch-back for time t₁ the depression over the trenchremains constant even though the layer is now thinner. After anothertime t₂, the depressed array nearly extends into the trench, despite thefact that the silicon mesa is not yet clear of oxide. After anadditional time t₃, the mesa has been cleared of the deposited glass butonly a small amount of the deposited oxide remains atop the trench gate.The dielectric thickness is thinnest in the center of the trench andremains thicker adjacent to the trench sidewalls, resulting in much lessplanarization than might otherwise be expected using etch-back methods.This etch-back process results in all or part of top oxide 84 shown inFIG. 2D.

[0012] Later oxide 90 is deposited, masked and etched by a contact maskand subsequently flowed at a high temperature to round its shape. Aftercompleting contact mask metallization 87 completes the structure.

[0013] As shown in the drawing of FIG. 2D, several problem areas mayoccur in such a device. Curvature at the bottom of the trench in region100, for example, may lead to gate oxide thinning and low gate breakdownvoltages. Using only aluminum as the metallization, metal spike 102 mayalloy through the N+ junction 86 and producing undesirable junctionleakage. Furthermore, polysilicon gate 83 must vertically overlap thebottom of N+ source 86 in region 102 or the MOSFET will not operateproperly (causing high on-resistance and possible loss offunctionality).

[0014] In FIG. 2E, a TiN layer 88 is deposited after the contact maskbut prior to deposition of metal layer 87. The titanium nitride layerblocks the aluminum metallization from alloying through the junctionthereby eliminating metal spiking. If the step height of the oxide cap90 atop the trench is too steep, the TiN layer 88 may crack, and spike104 may result. In each case shown, poor planarization leads to defects,poor uniformity, and yield loss.

[0015] The problem of topography complicating planarization is furtherillustrated in FIG. 3A, which shows a cross section 120 of an activetrench array of trenches 123A, 123B, and 123C along with gate bus area124, just after polysilicon deposition and partial etch-back producing apolysilicon gate bus 131G of thickness x atop oxide 125. If thickness xis minimized to prevent step height related step coverage problemslater, the depth of the surface depressions 135A, 135B, and 135C at thetop of the active polysilicon trench gates will be extreme and may bedifficult to fill later.

[0016] Cross section 160 shown in FIG. 3B illustrates the contrastingcase where the polysilicon layer of thickness x is sufficiently thick toexhibit a more planar surface over the trenches, whereby depressions162A, 162B, and 162C are at a minimum. FIG. 3C illustrates a benefit ofusing very thick polysilicon in device 160 is the relatively goodplanarization results and uniform etch-back of embedded polysiliconregions 166A and 166B. Thick polysilicon, e.g., over 1 to 1.5 μm thick,requires long processing time, adversely adding to wafer costs. Moresignificantly, another problem with such a thick polysilicon layer isalso revealed in FIG. 3C, where polysilicon gate bus 165 comprisingembedded portion 166C and surface portion 167 produces two extreme stepsin the wafer's topography: step 180A over active areas and step 180B ontop of field oxide 181. Either location may later cause problems withglass depositions, TiN metallization, or metal breakage.

[0017]FIGS. 4A to 4J illustrate the fabrication sequence of atrench-gated MOSFET with a self-aligned contact as disclosed in U.S.Pat. No. 6,413,822, entitled “Super-Self-Aligned Fabrication Process OfTrench-Gate DMOS With Overlying Device Layer”, which is herebyincorporated by reference in its entirety. The self-aligned contactmethod allows the contact feature to be opened over the entire activedevice array device without leading to gate-to-source shorts. Thisfeature allows improved body contact and/or denser trench array cellfeatures and smaller devices. In the fabrication sequence, FIG. 4Aillustrates MOSFET trench cell region 200 in which a composite hard maskcomprising oxide 205, silicon nitride 206, and top oxide 207 ispatterned by photolithographic means and then used to act as a hard maskfor silicon trench etching to produce trench feature 201. Gate oxide 204is then formed after appropriate sacrificial oxidations. Unlike in othertrench fabrication sequences, in the sequence beginning in FIG. 4A, thehard mask remains on the silicon during subsequent processing. In FIG.4B, thick bottom oxide 211 is deposited directionally using CVD.Additional oxide 210 then forms atop the hard mask during saiddirectional deposition with little or no deposition on the trenchsidewall.

[0018] In FIG. 4C, polysilicon 215 is deposited to fill the trench to athickness that overflows the trench and deposits above the siliconsurface. The polysilicon overflow is then etched back to form embeddedpolysilicon 215 as shown in FIG. 4D, top oxides 210 and 207 are removedas shown in FIG. 4E, and a P-type body region is implanted throughnitride 206 into the silicon mesa regions interposing the trenches asshown in FIG. 4F. The body may be implanted at a shallow depth with alow energy ion implant (i.e., with under 150-keV boron ions) thendiffused to a depth of substantially 85% of the trench depth, oralternatively, the body may be entirely implanted using a chain-implantof multiple boron implantations of differing energies (ranging from upto 2 MeV for the deepest portions to 150 keV for the shallowestportions) all through a common mask opening.

[0019] After the body implant, in steps not shown, a second polysiliconlayer is deposited, masked, and etched back to leave polysilicon only inthe surface gate bus regions and where the trench polysilicon connectsto the polysilicon gate bus. After the second polysilicon etch-back,thermal oxide 226A is grown followed by glass or BPSG deposition ofdielectric 225A as shown in FIG. 4G. In FIG. 4H, dielectric 225A isetched back below the surface of original nitride hard mask 206.Thereafter, nitride hard mask 206 is removed to result in the structureof FIG. 41 where only a small portion of glass 225A remains above thetrench. In device 240 of FIG. 2J which represents an array of activetrench devices 220, the contact mask (needed to contact gate bus regionsnot shown in the cross section) opens all the active areas to thecontact etch but removes minimal amounts of oxide so as to avoid shortsto embedded polysilicon 215. Thereafter, TiN 230 and top metal 231 aredeposited, patterned, and etched.

[0020] In FIG. 5A, the gate bus region of the same device is illustratedafter the second polysilicon deposition and masked etch back.Polysilicon gate bus 226B remains on the surface of the device whileonly a small portion of the second polysilicon 226A may survive etchback inside the trench itself. The final polysilicon etch backuniformity is critical since any polysilicon protruding from a trenchwill short the device's gate to its source metal. Only etch back methodsmay be employed to planarize the polysilicon since the surface of thedevice is not planar, having various steps, and topographic featuresabove and below the wafer's surface. Such topography prevents the use ofchemical mechanical polishing (CMP) techniques, since mechanicalplanarization of the trench polysilicon would remove the gate busentirely from the device's surface.

[0021] With such uneven topography, the need for glass 225A (shown afterits deposition in FIG. 5B) becomes tantamount for preventing stepcoverage problems. The glass 225A is flowed as illustrated in FIG. 5Cusing a short yet relatively high-temperature furnace operation(typically 900° C. for 15 minutes) and patterned as shown in FIG. 5Daccording to a contact mask. FIG. 5D illustrates the contact to the gatebus (above the silicon surface), and the large self-aligned contact openacross the top of all the active trench cells. The contact maskingoperation is complicated by the two distinct heights of the active arrayand the gate bus regions, but since no critical feature is presentwithin the cell array, the contact etch operation is possible providedacross-the-wafer uniformity is not at issue.

[0022] In FIG. 5E, TiN barrier metal is deposited to a thicknesssufficient to prevent metal spiking in the contact windows and to coverthe topographical steps, but thin enough not to crack from film stress.In FIG. 5F the thick metal, typically Al—Cu or Al—Cu—Si is deposited toa thickness of about 3 μm, Masked and etched. Metal 270A represents thesource metal, where 270B is the gate contact or gate bus.

[0023]FIG. 6A illustrates a plan view of a trench gated power MOSFEThaving a source metal electrode 283, a metalized gate pad 280, and agate bus 281. A gap 282 separates the two metals and prevents shorting.The drain contact is made to the wafer's backside. FIG. 6B illustratesanother trench MOSFET, having three separate source metals 285A, 285Band 285C, broken into islands by gate metal comprising metal gate ring286A and busses 286B and 286C connected to metalized bonding pad 284. Insuch a device the separate source regions are not electrically shorteduntil wire bonding and packaging.

[0024] Device areas can be substantial with dimensions spanning severalmillimeters across a die. Uniformity across a die and across a wafer cantherefore be problematic in achieving highly manufacturable products.Typical uniformity issues can manifest themselves as irregular andrandom metal voids such as 300A and 300B in contact openings of a device290 of FIG. 7A. Even in a MOSFET 310 having self-aligned contacts asshown in FIG. 7B, polysilicon deposition, oxidation and etch back canlead to “horns” and other irregular etch features like the top ofpolysilicon gate 314B and voids as shown in gate region 314C.

[0025] Topography can also lead to steps coverage and depth of focusproblems for photolithography, including potential gate shorting inregion 350, TiN cracking (region 351 and 353), and metal thinning andreentrant angles over steep steps 352 and 354 as shown in FIG. 7C. Allthese failures may compromise the performance, yield, or reliability ofa trench MOSFET. The problems are greatly exaggerated by the non-planarsurface of the device, requiring photolithographic-masking, etching,etch backs, planarization, and depositions to behave similarly (oridentically) on different heights within a single device.

[0026] Not only does the extreme topography of today's vertical trenchpower MOSFET processes limit the device's cell density and performance,its high intrinsic gate resistance limits its switching speed, mandatingthe need for additional metalized gate bus regions. The schematic ofFIG. 8A, representing an array of MOSFETs can be used to examine howdistributed gate resistance affects the switching speed of large areapower MOSFETs. The lumped element of a single three-terminal powerMOSFET 360 comprises an array of identical MOSFETs 362, wherebytransistors M₁ through M_(n) are connected with their source-drainterminals in a parallel network configuration sharing a common sourceelectrode 366 and a common drain electrode 368. Although the devicesalso share a common gate connection 365, the gate resistance is notconstant for the various devices. Devices near a gate bus, e.g. M₁,conduct the gate signal through a small series gate resistance of r_(g1)where device M₂ exhibits a higher series gate resistance of(r_(g1)+r_(g2)). With a higher gate resistance, the more remote deviceswitches slower than those devices located nearest the gate bus. CellM_(n), farthest from the gate bus having a series gate resistance of(r_(g1)+r_(g2)+ . . . +r_(gn)) can suffer serious gate propagationdelays, even switching ten times slower than cells near the metalizedgate bus. So the farthest device from the metalized gate bus is slowestin both turn-on and turn-off transients, being the last device to turnon during a turn-on transient, and the last device to turn off during aturn-off transient. As illustrated in plot 370 of FIG. 8B, driving gateinput signal 371 from “on” to “off”, and then to “on” again causes draincurrent through a resistive load to roughly “follow” the input waveformafter some time delay. In ideal case 373, the drain current changesquickly after the input transition. In drain current waveform 372, whichincludes distributed gate resistance, the device experiences a turn-offdelay t_(d)(Off) followed by a slow fall in current I_(D) for durationt_(fall), the fall time of the device. Turn on has a similar responsewith a delay t_(d)(on) followed by a slow ramp up in current forduration t_(rise), the rise time of the device. During slow transients,the simultaneous presence of both voltage across the device and currentthrough the device greatly increases power losses in the device andlowers the efficiency of using such a switch in many applications. Theeffective gate resistance can be reduced by including gate bussesregularly and more frequently throughout the device, but only bysacrificing area from active device arrays for gate bussing. More gatebussing and less active cells increases the switch's on-resistance ordie size, and therefore cost. Moreover the gate bus regions exacerbatethe non-planar topography issues that further limit device density andperformance improvements.

[0027] What is needed is a vertical trench-gated power MOSFET capable ofintegrating large arrays of active vertical MOSFETs at high densitieswith integral gate bus and gate contact structures in an area efficientdevice having a relatively flat or planar surface topography. Ideally,the device should offer the lowest possible series gate resistance forfast switching capability, and exhibit low drain-to-source area-specificon-resistance. Finally, the fabrication of such a device shouldaccommodate processes for achieving better film planarization uniformitythan that of standard etch back methods.

SUMMARY OF INVENTION

[0028] A trench-gated vertical power MOSFET and fabrication processesfor such power MOSFETs use a continuous conductive gate structure withintrenches to avoid problems that topology causes when a gate bus extendsabove a substrate surface. The conductive gate structure forms gates indevice trenches in an active device region of the power MOSFET and formsa gate bus in a gate bus trench. The gate bus trench that connects tothe device trenches can be wide to facilitate forming a gate contact tothe gate bus, while the device trenches are narrow to maximize devicedensity. CMP processes can be used to planarize the conductive gatestructure and/or overlying insulating layers, and the processes arecompatible with processes forming self-aligned or conventional contactsin the active device region.

[0029] In one specific embodiment of the invention, a power MOSFETincludes a substrate having a gate bus trench and one or more devicetrenches and extending from a top surface of the substrate. In each mesaadjacent and/or between the device trenches, a source region, a channelregion, and a drain region are arranged vertically along at least aportion of a wall of the device trench next to the mesa. A gatestructure extends continuously in the device and gate bus trenches, andthe top surface of the gate structure does not extend above the topsurface of the substrate. Portions of the gate structure in the devicetrenches act as a gate of vertical devices in the wall of the trench,and a portion of the gate structure in the gate bus trench forms a gatebus. A gate contact contacts the gate bus in the gate bus trench. Thegate bus trench is generally made wider than the device trenches tofacilitate creation of the gate contact while still maximizing thedensity of vertical devices. Also, the device trenches can be deeperthan the gate bus trench and the substrate under the gate bus trench canbe doped to reduce gate-to-drain or gate-to-source capacitance.

[0030] Power MOSFETs in accordance with alternative embodiments of theinvention can employ a variety of contact structures. Conventionalnon-self-aligned gate and source contacts can be formed through aplanarized insulating layer to the gate bus in the second trench andsource regions in the substrate. The planar topology provided by CMPprocesses that the in-trench gate structure allows facilitates accurateetching of contact openings.

[0031] In an architecture having self-aligned source contacts, aninsulating material residing in the trenches above the gate structure,has a top surface of the insulating material that is coplanar with thetop surface of the substrate. A metal contact layer formed on the planarsurface of the substrate and the insulating material includes a sourcecontact region that contacts the source region of the vertical deviceformed in the substrate's mesa. The insulating material in the trenchesprevents the source contact region from shorting to the gate structure.A gate contact region of the metal layer contacts the gate bus throughan opening formed through the insulating material in the gate bustrench.

[0032] In another self-aligned contact structure, insulating regionsextend above the trenches. These insulating regions remain from a CMPplanarization that removes insulating material down to a hardmask thatwas used to define the boundaries of the trenches. After theplanarization and removal of the hardmask, insulating regions remain andcan be flowed to improve insulation between an overlying metal sourcecontact region and the gate structure in the device trenches.

[0033] The conductive gate structure generally includes a continuouspolysilicon region that extends from the device trenches into the gatebus trench. Additionally, metal or silicide can be deposited in the gatebus trench, the device trenches, or both. Deposition of a firstpolysilicon layer before and after deposition of a high-temperaturemetal can enclose the metal or silicide to prevent metal diffusion intothe gate oxide in the device trenches. Alternatively, the metal orsilicide can directly contact oxide in the trenches if only lowertemperature processes are employed after deposition of the metal.

[0034] Another specific embodiment of the invention is a fabricationprocess for a power MOSFET. Generally, the fabrication process includesdevice and gate bus trenches in a substrate, which may include anepitaxial layer. A conductive gate structure is then formed that extendscontinuously from the device trenches into the gate bus trench. Dopingof a mesa in the substrate adjacent to a device trench forms a verticaldevice including a source region, a channel region, and a drain regionthat are vertically aligned along a wall of the device trench. Theprocess creates a structure including the conductive gate structure inthe trenches and permits a chemical mechanical polishing (CMP) of thestructure. Thus, formation of a contact layer is after the CMP process,which improves the reliability of the source contact to the verticaldevices and the gate contact to the gate structure in the gate bustrench. The gate bus trench is generally shallower and wider than thedevice trenches.

[0035] A variety of processes for trench formation are available. Oneembodiment of the invention uses LOCOS to oxidize the substrate in thearea of gate bus trench, and the gate bus trench remains after removingan oxide region formed by LOCOS in the substrate. Alternatively, thedevice and gate bus trenches can be formed via separate mask and etchprocesses.

[0036] The process of forming contacts generally includes forming aninsulating layer overlying the conductive gate structure. A contact etchcan create an opening through the insulating layer to expose a portionof the conductive gate structure in the gate bus trench, and a gatecontact region filling the opening contacts the exposed portion of thegate structure in the second trench. The contact etch can also form asecond opening through the insulating layer that exposes the mesa, sothat a source contact region filling the second opening contacts thesources of the vertical devices. In an alternative embodiment, the CMPremoves the insulating layer from above the mesa, so that depositing ametal contact layer on the planarized surface creates self-align sourcecontacts.

[0037] In yet another embodiment of the invention, a hardmask havingopenings that define boundaries of the first and second trenchesprovides an etch stop for the CMP process. The CMP process exposesregions of the hardmask overlying the mesas in the active device regionand leaves the regions of the insulating layer in the openings in thehardmask. Removing the hardmask without removing the regions of theinsulating layer left after the CMP process exposes the mesa whileleaving additional insulating material protecting the gate structure inthe device trenches. The contact layer thus contacts the mesa where theregions of the hardmask were removed and the regions of the insulatinglayer separate the gate structure in the device trenches from thecontact layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIGS. 1A to 1H are cross-sectional views illustrating afabrication sequence of a conventional vertical trench-gated powerMOSFET (active device array only), in which FIG. 1A illustrates a trenchmask, FIG. 1B illustrates post trench polysilicon fill, FIG. 1Cillustrates polysilicon etch back, FIG. 1D illustrates shallow body ionimplantation, FIG. 1E illustrates body diffusion and source ionimplantation, FIG. 1F illustrates glass deposition and contact mask,FIG. 1G illustrates post-metallization (for a single cell), and FIG. 1Hillustrates post metallization (array of cells).

[0039]FIGS. 2A to 2E are cross-sectional views illustratingcomplications in manufacturing trench gated power MOSFETs, in particularFIG. 2A shows the complexity of gate bus topography and polysilicon etchback into trench, FIG. 2B illustrates trench uniformity issues, FIG. 2Cillustrates trench top oxide etch back uniformity issues, FIG. 2Dillustrates contact metal spiking and oxide defects, and FIG. 2Eillustrates titanium nitride cracking and metal spiking.

[0040]FIGS. 3A to 3C illustrate issues in planarization of both activeand gate bus regions in conventional trench gated power MOSFETs, inparticular FIG. 3A illustrates poor trench fill resulting frominadequate polysilicon trench fill process, FIG. 3B illustrates improvedtrench fill using thick polysilicon deposition, and FIG. 3C illustratesstep-height problem of thick polysilicon trench refill.

[0041]FIGS. 4A to 4J illustrate a fabrication sequence for ahigh-density-capable vertical trench-gated power MOSFET withself-aligned contact (active device array only), in which FIG. 4Aillustrates a trench etch with a hard-mask, FIG. 4B illustratesdirectional deposition of a thick bottom oxide, FIG. 4C illustrates afirst polysilicon trench fill, FIG. 4D illustrates a first polysiliconetch-back, FIG. 4E thick bottom oxide top oxide removal, FIG. 4F thickbottom oxide a diffusion-less chained body implant using multiplehigh-energy ion implantations, FIG. 4G illustrates a source implant andglass deposition, FIG. 4H illustrates a glass etch-back, FIG. 4Iillustrates removal of the top nitride hard-mask, and FIG. 4Jillustrates contact and metallization in the active array of devices.

[0042]FIGS. 5A to 5F illustrate the fabrication of a gate bus region ina self-aligned-contact version of a trench gated power MOSFET, inparticular FIG. 5A illustrates an N+ implant after a second polysilicondeposition and masked etch-back, FIG. 5B illustrates glass deposition,FIG. 5C shows the result glass flow, FIG. 5D shows the result of acontact mask etch, FIG. 5E illustrates a nitride strip and TiN barrierdeposition, and FIG. 5F illustrates metal deposition, masking, andetching.

[0043]FIGS. 6A and 6B respectively show plan views of a vertical powerMOSFET having a single gate finger device and gate bonding pad and avertical power MOSFET having multi-source islands with a gate bus gridsurrounding the source islands.

[0044]FIGS. 7A to 7C illustrate uniformity issues in trench gate MOSFETproduction, FIG. 7A showing metal voiding in active array contactwindows, FIG. 7B illustrating polysilicon deposition voids and etch-backuniformity issues, and FIG. 7C illustrating metal step coverage andbreakage issues

[0045]FIGS. 8A and 8B illustrate gate propagation issues in verticalpower MOSFETs, FIG. 8A showing equivalent schematic of a vertical MOSFETarray with distributed gate resistance, and FIG. 8B showing a slowswitching waveform of device with high gate resistance.

[0046]FIGS. 9A and 9B are cross sectional views of vertical trench-gatedpower MOSFET with co-planar gate bus requiring “ideal” contact placementrespectively in a vertical trench-gated MOSFET with contact mask featurein the active cell array and a vertical trench-gated power MOSFET withself-aligned contact across active cell array.

[0047]FIG. 10 is a cross sectional view of a vertical trench-gatedMOSFET with a co-planar buried gate bus and self-aligned contacts withinthe active cell array

[0048]FIGS. 11A to 11Q are cross sectional views illustratingfabrication of a vertical power MOSFET with a co-planar gate bus andself-aligned contacts across the active cell array, FIG. 11Aillustrating buried-gate-bus hard mask definition, FIG. 11B illustratinga buried-gate-bus shallow trench etch, FIG. 11C illustrating active-areatrench mask definition, FIG. 11D illustrating an active-area trenchetch, FIG. 11E showing trench sacrificial oxide, FIG. 11F illustratingthick bottom oxide formation, FIG. 11G illustrating a first polysilicondeposition, FIG. 11H illustrating the results of chemical mechanicalpolishing or a polysilicon etch-back, FIG. 11I illustrating apolysilicon over etch, FIG. 11J illustrating thin top oxidation, FIG.11K illustrating CVD oxide formation, FIG. 11L illustrating CMPdielectric planarization, FIG. 11M illustrating a gate contact mask,FIG. 11N illustrating a gate contact etch, FIG. 11O illustrating anactive (mesa) nitride strip, FIG. 11P illustrating an oxide etch (ordip), and FIG. 11Q illustrating metallization and a metal mask.

[0049]FIG. 12 shows a three-dimensional projection of coplanarpolysilicon structure forming trench gates and a buried-gate bus.

[0050]FIGS. 13A to 13G are cross sectional views illustrating of anoptional process flow with CMP removing all dielectrics located abovethe silicon surface, FIG. 13A showing a structure after glassdeposition, FIG. 13B showing the structure after a planarizing CMP, FIG.13C illustrating oxide deposition, FIG. 13D illustrating a gate contactmask, FIG. 13E illustrating a contact etch, FIG. 13F illustrating CMPand/or etch-back planarization, and FIG. 13G illustrating themetallization.

[0051]FIGS. 14A to 14P are cross sectional views illustrating analternate process flow for fabrication of a vertical trench gatedMOSFET, FIG. 14A illustrating buried gate bus mask definition, FIG. 14Billustrating wide trench etch and trench gate mask definition, FIG. 14Cshowing the structure after narrow trench etch and mask removal, FIG.14D illustrating sacrificial oxide, oxide strip, and gate oxidation,FIG. 14E illustrating a polysilicon fill process, FIG. 14F illustratingCMP planarization, FIG. 14G illustrating polysilicon over-etch, FIG. 14Hillustrating thin top oxidation, FIG. 14I illustrating glass deposition,FIG. 14J illustrating CMP and/or etch-back for dielectric planarization,FIG. 14K illustrating a contact mask having a single contact for bothgate and active mesa regions, FIG. 14L illustrating a contact etch andoptional glass flow (for a single contact process), FIG. 14Millustrating TiN (barrier metal) and Al—Cu—Si thick metal deposition,mask and etch (for a single contact process), FIG. 14N illustrating agate contact mask and etch (for a dual contact mask sequence), FIG. 140illustrating an active array contact mask and etch (for a dual contactmask sequence), and FIG. 14P illustrating TiN (barrier metal) andAl—Cu—Si thick metal deposition, masking, and etching (for a dualcontact mask sequence).

[0052]FIGS. 15A to 151 are cross sectional view illustrating of analternative process flow for fabrication of a vertical trench gatedMOSFET with capping dielectric, FIG. 15A illustrating wide and narrowtrenches after thick bottom oxide deposition, FIG. 15B showing thestructure after sacrificial sidewall oxide removal and gate oxidation,FIG. 15C illustrating polysilicon fill, FIG. 15D illustrating a CMPplanarization, FIG. 15E illustrating capping dielectric deposition(e.g., of Si₃N₄), FIG. 15F illustrating a planarizing etch-back and/orCMP of the capping dielectric, FIG. 15G illustrating a gate contactmask, FIG. 15H showing the structure after a gate contact etch (andphotoresist strip), and FIG. 15I illustrating TiN (barrier metal) andAl—Cu—Si thick metal deposition, masking and etching.

[0053]FIGS. 16A to 16H are cross sectional view illustrating a processflow for fabrication of a vertical trench gated MOSFET with buriedsilicided gate, FIG. 16A illustrating gate oxidation after thick bottomoxide directional CVD in wide and narrow trench structures, FIG. 16Billustrating a first polysilicon (lining) deposition, FIG. 16Cillustrating a high-temperature metal deposition (e.g., of titanium ortungsten), FIG. 16D illustrating a metal etch-back, FIG. 16Eillustrating a second polysilicon (sealer) deposition, FIG. 16Fillustrating CMP planarization and silicide reaction, FIG. 16Gillustrating polysilicon etch-back, top oxide formation andplanarization, and FIG. 16H illustrating gate contact masking andbarrier metal and Al—Cu—Si thick metal deposition and masking

[0054]FIG. 17 is a perspective view of a polysilicon trench gate andburied gate bus with embedded silicide.

[0055]FIGS. 18A to 18D are cross sectional views illustrating dopantintroduction steps compatible with a fabrication process for a trenchgated MOSFET having buried gate bus, FIG. 18A illustrating a P-type wellimplanted into wide trench (below a gate bus), FIG. 18B illustratingactive mesa P-type body formation using masked chained implant method,FIG. 18C illustrating source mask and N+ ion implantation, and FIG. 18Dillustrating a shallow P+ mask and implant process.

[0056]FIGS. 19A to 19D illustrate wide trench formation using an etchedLOCOS technique, FIG. 19A illustrating a wide trench mask and Si₃N₄etch, FIG. 19B illustrating a thick field LOCOS, FIG. 19C illustratingnarrow trench mask definition and etching, and FIG. 19D illustrating anitride and oxide strip.

[0057]FIGS. 20A to 20E illustrate alternative wide trench formationprocesses using etched LOCOS technique, FIG. 20A illustrating a widetrench mask and Si₃N₄ etch, FIG. 20B illustrating a thick field LOCOS,FIG. 20C illustrating a LOCOS oxide strip, FIG. 20D illustrating nitrideoverhang etch and P-type well masked and implanted into a wide trench,and FIG. 20E illustrating a narrow trench mask definition and etching.

[0058]FIGS. 21A to 211 are cross sectional views illustrating processsteps for wide trench fill and buried polysilicon gate bus formation,FIG. 21A illustrating a wide trench after P-type well formation, FIG.21B illustrating wide trench oxidation, FIG. 21C illustratingpolysilicon deposition, FIG. 21D illustrating CMP planarization, FIG.21E illustrating narrow trench mask definition and etching, FIG. 21Fillustrating narrow trench polysilicon fill with optional embeddedsilicide comprising a first polysilicon (lining) deposition, a hightemperature metal deposition, and a second polysilicon (sealant)deposition, FIG. 21G illustrating CMP planarization, FIG. 21Hillustrating trench fill etch-back and top dielectric deposition, andFIG. 21I illustrating CMP planarization.

[0059]FIGS. 22A to 22L are cross sectional views illustratingalternative process steps for wide trench fill and buried polysilicongate bus formation including embedded silicide, FIG. 22A illustrating awide trench (bus area) after oxidation, a first polysilicon (lining)deposition, and a high temperature metal deposition, FIG. 22Billustrating a high-temperature metal etch-back, FIG. 22C illustrating asecond polysilicon (sealant) deposition, FIG. 22D illustrating CMPplanarization of a gate bus, FIG. 22E illustrating narrow trench maskdefinition and trench etching, FIG. 22F illustrating thick bottom oxidedeposition, FIG. 22G illustrating a third polysilicon (narrow trenchlining) deposition followed by high temperature metal deposition, FIG.22H illustrating a high temperature metal etch-back and a fourthpolysilicon deposition (narrow trench sealant), FIG. 22I illustratingCMP planarization, FIG. 22J illustrating gate etch-back and topdielectric deposition, CMP and/or etch-back planarization, and overetch, FIG. 22K illustrating nitride lift-off, and FIG. 22L illustratinga gate contact mask and etching, TiN barrier metal, Al—Cu—Si thickmetal, metal mask and etching.

[0060]FIGS. 23A to 23H are cross sectional views of trench gated MOSFETcells having embedded silicide gate, FIG. 23A illustrating a cellwithout thick bottom oxide but with silicide enclosed by polysilicon,FIG. 23B illustrating a cell with thick bottom oxide in trench andsilicide enclosed by polysilicon, FIG. 23C illustrating a cell withoutthick bottom oxide but with a gate oxide lined by polysilicon to preventdirect contact to embedded metal, FIG. 23D illustrating a cell withthick bottom oxide and gate oxide lined by polysilicon to prevent directcontact to embedded metal, FIG. 23E illustrating a cell without thickbottom oxide but with a bamboo (layered) gate with embedded metal attrench bottom, polysilicon in gate/channel region, and a dielectric cap(no polysilicon sealer to prevent direct contact to embedded metal),FIG. 23F illustrating a cell with thick bottom oxide, a bamboo (layered)gate with embedded metal at trench bottom, polysilicon in gate/channelregion and dielectric cap (no polysilicon sealer to prevent directcontact to embedded metal), FIG. 23G illustrating a cell without thickbottom oxide but with an alternative bamboo (layered) gate withpolysilicon in gate/channel region, embedded metal above polysilicongate, and a dielectric cap (no polysilicon sealer to prevent directcontact to embedded metal), and FIG. 23H illustrating a cell with thickbottom oxide, the alternative bamboo (layered) gate with polysilicon ingate/channel region, embedded metal above polysilicon gate, anddielectric cap (no polysilicon sealer to prevent direct contact toembedded metal).

[0061]FIG. 24 is a flow chart of processes for fabrication of variantsof trench gate MOSFET with a wide buried gate bus and narrow trenchgates.

[0062]FIG. 25 is a flow chart of processes for fabrication of variantsof trench gate MOSFETs with embedded silicide trench gates and/or asilicided buried gate bus.

[0063]FIG. 26 is a flow chart of processes used to produce a wide buriedgate bus using etched LOCOS technique.

[0064]FIG. 27 is a flow chart of processes used to produce gatestructures including metal/silicide to improve conductivity.

DETAILED DESCRIPTION OF INVENTION

[0065] As shown in FIG. 9A, a trench-gated vertical power MOSFET (alsoknown as a trench DMOS device) associated with this invention comprisesan array of tightly packed devices in an array of stripes or polygonalcells (the portion located beneath metal 407A in the drawing) having aseries of trenches 403A, 403B, 403C etched into the silicon surface,lined with a thin gate oxide 399A, 399B, 399C in all (or along someportion) of the trenches, and filled with a conductor such asheavily-doped polysilicon 404A, 404B, 404C. The embedded polysilicon isnot contacted within this “active cell array”, but instead is contactedby extending the trench with its embedded polysilicon into anotherregion to contact the trench polysilicon with metal. Between trencheslies a mesa region typically used to integrate the MOSFET's source, body(channel), and body contact features. Within the active array, the mesaregion includes contact windows 408A, 408B, 408C, and 408D to facilitateelectrical contact with the MOSFET's source and body elements withoutshorting to the embedded gates. These contacts form where the siliconsurface comes in contact with the thick Al—Cu Si metallization 407A. Theinterface of the contact may also include barrier metal 406A comprisingTiN to reduce contact resistance and prevent aluminum diffusion into thesilicon (contact spiking). In the cross section shown, additionaldielectric regions 405A, 405B, 405C sit atop the active trench array toprevent accidental shorting between the top metal 407A and the embeddedpolysilicon gates.

[0066] In another region of the device, a trench 403D is used toimplement a contact between the gate metal 407B and the embeddedpolysilicon array 404D. As described in the background section of thisdisclosure, such a contact is normally facilitated by extending thepolysilicon beyond the trench and up onto a dielectric layer coveringthe silicon surface. Such a multi-level structure suffers from manyprocess and structural limitations as enumerated in the aforementionedbackground section of this application.

[0067] In the cross section of FIG. 9A, contact to the gate is notimplemented by extending the polysilicon outside of the trench as inconventional vertical power MOSFETs, but instead by extending thecontact down into trench 403D, which is dedicated for gate contact. Insuch a structure, contact 409 must be perfectly placed atop gatepolysilicon 404D, so that etching through the oxides 405D and 405E willcontact the polysilicon without shorting to any silicon mesa regionoutside the trench. The alignment and size of the contact (with allmanufacturing variations considered) cannot allow the contact mask tofall off the polysilicon 404D and overlap onto gate oxide 399D or thegate oxide will be attacked by the etch and a short to the mesa willunfortunately result.

[0068] Topographically, however, the contact to polysilicon 404D isideal since this layer is coplanar, i.e. at the same vertical height, asthe surface of the contacts to the various silicon mesa regions. Putanother way, the gate bus is substantially coplanar with the activearray because gate contact 409 and mesa contacts 408A, 408B, 408C, and408D geometrically lie in the same plane.

[0069] To implement the “ideal” gate contact 409, its alignment andfeature size require much greater precision than do the mesa contacts408. If contact dedicated trench 403D is etched to the same size as theactive array trenches 403A, 403B, and 403C, making gate contact 409 isextremely difficult using processing equipment commonly available in0.25 μm wafer fabs. Contact features smaller than 0.1 μm may berequired. Such ultra-advanced fabs are unfortunately too expensive tomanufacture such discrete power devices in a cost effective manner.

[0070] One remedy to this problem is to widen trench 403D to a widthwider than active array trenches 403A, 403B, and 403C. For example, ifactive trenches are patterned to a width of 0.2 μm or 0.35 μm, and gatecontact trench 403D is patterned to a width of 0.9 μm, then contact 409could theoretically employ a 0.25 μm feature size. Beyond about 1 μm ofwidth the contact trench 409 becomes much more difficult to fill andplanarize. Even so it remains difficult to produce trenches of widelydiffering widths in a single photolithographic step. To improve matters,a trench used for gate contact (or gate bussing) may be masked andetched at a different time than those of the active array so thatpatterning, etching, filling, and planarization may be independentlyoptimized.

[0071] Aside from the need for a small-sized perfectly-aligned gatecontact, the vertical trench-gated MOSFET of FIG. 9A illustrates thatcoplanar contacts for gate and active cells are compatible with devicesemploying a contact feature in the active cell array. In a similarfashion, FIG. 9B illustrates a vertical trench-gated power MOSFET with aself-aligned contact 420 (comprising mesa contacts 420A, 420B, 420C, and420D) which may be implemented using a process as described in U.S. Pat.No. 6,413,822, which was incorporated by reference herein above. Suchdevices with a self-aligned contact running across the top of the activecell array can still benefit from a coplanar gate contact 421. In thisexample, the top of gate polysilicon 405D is located slightly below thesurface of (but still substantially co-planar to) the silicon mesacontacts 420.

[0072] In both examples shown, using a trench for gate contact (or gatebussing) that is masked and etched independently from the processing ofthe active trench array allows complete flexibility in optimization ofgate and active trenches. Since gate busses (or trench-gate contacts) donot determine the conduction characteristics of the device, theirassociated trench depth need not match that of active device arraytrenches.

[0073] Accordingly, many variants of improved vertical trench gatedMOSFETs with a planarized gate bus can be fabricated using two (or more)trench-etching processing operations (instead of one homogeneous trenchthroughout). As it will be shown, the key process steps in fabricatingan improved trench MOSFET with a planarized gate bus involve methods toform two distinct trenches and to fill and planarize these trenches. Theformation of doped areas by ion implantation and their associateddiffusions (when applicable) to create the MOSFET's body, source, andbody contact regions may be combined with these multi-trench structuresin a myriad of combinational sequences, many resulting in similar finalstructures. Some examples of vertical trench gated MOSFETs combiningboth trench formation and junction formation are included for clarity.Otherwise, the numerous process sequences shown herein often skip thesteps of dopant introduction in the cross-sectional diagrams of thedevices, such steps being well known to those skilled in the art.

[0074]FIG. 10 illustrates a cross section of a vertical trench-gatedMOSFET 430 in accordance with an embodiment of the invention comprisinga co-planar wide (buried) gate bus 439 and self-aligned contacts 445A to445E within the active cell array of narrow trench devices. The surfaceof the contacts is planarized flat using chemical mechanical polishing(CMP) or other means during the device fabrication so that virtually notopography exists when contact and metallization steps are performed. Asshown, device 430 comprises a series of narrow trench gate structures433A, 433B, 433C, 433D and 433E in the active array region of the device(i.e., the portions of the device where the vertical MOSFET source,body, and channel are formed) including embedded polysilicon gates 438A,438B, 438C, 438D and 438E, thick bottom oxide 435A, 435B, 435C, 435D and435E located at the bottom of each narrow trench, and top oxide 444A,444B, 444C, 444D and 444E protecting the top of each trench embeddedpolysilicon from shorting to the overlying source metal 441A or 441C.Source metal 441A contacts silicon mesa regions 445A, 445B, 445C and445D via barrier metal 440A which may contain a variety of P-type andN-type doped regions (not shown), including N+ source, P-body, and P+contact region to the P-body. These doped regions may be formed by ionimplantation (or ion implantations combined with high-temperaturethermal diffusions). The doped regions, along with the gate bus and theactive trench array are formed within N epitaxial layer 432 atop heavilydoped N+ substrate 431. Similarly, source metal 441C contacts siliconmesa regions 445E and 445F via barrier metal 440C. Source metals 441Aand 441C are typically electrically shorted by aluminum metallization orby one or more bond wires.

[0075] In wide trench 434, metal layer 441B contacts polysilicon gatebus 439, which is isolated from epitaxial layer 432 (or any other dopedregions) by thick oxide 436. Gate bus polysilicon 439 forms a continuousgrid of polysilicon with narrow trench embedded polysilicon 438A through438E. The purpose of the polysilicon contact and metal region 441B is tofacilitate electrical connections to the MOSFET's gate (via bond wire)and to provide a means to distribute electrical signals to the gate ofthe device across a chip's surface without significant propagationdelays. Signals travel from the metal gate connection 441B into the gatebus polysilicon 439 and then into the array of narrow trench-enclosedpolysilicon gate 438. The gate bus structure may be repeated at regularintervals if high switching speed devices are required.

[0076] What is noteworthy of this device structure is its extremelyplanar surface making photolithography and formation of contacts andmetallization 441A, 441B, and 441C much easier. With no oxide steps ortopography above the silicon surface, there is no risk of metalbreakage, step coverage induced metal thinning, metal voiding, orelectromigration at sensitive locations within the device. Inparticular, at no location in the device is it necessary for polysiliconto emerge from a trench onto the silicon surface, either in the activecell array (beneath metal 441A and 441C) or in the gate bus orgate-contact area itself (beneath metal 441B).

[0077] Another distinctive characteristic of this device is theco-planarity of the gate bus polysilicon 439 and the polysilicon 438(i.e., 438A through 438E) embedded within the trenches. The top of bothpolysilicon regions 439 and 438 are substantially planar, and do notextend outside of the confines of a trench. Even more unique is thepresence of two distinctly different shaped trenches, one (433) shallowand wide, the other (435) narrow and deeper. The shallow-wide trench 433comprises the gate bus and gate contact area of the device, having aminimum width that is easily contacted (e.g., greater than 1 μm wide).In a gate bus, however, more than a simple electrical contact to thegate is needed since the gate bus carries signals across the chip. Toavoid propagation delays, the series resistance of the gate bus must beheld to a minimum. Accordingly, the width of the gate bus may beexpanded to facilitate a low resistance metal trace, e.g., a metal bus10 μm to 25 μm wide. The depth of the gate bus may be roughly half thatof the active gate trench, e.g. 0.5 μm to 0.7 μm deep.

[0078] The active trenches 435 (i.e., 435A through 435E) are deeper,ranging from 1 μm to 2 μm in depth (with 1.5 μm being preferred). Unlikethe ideal gate bus, these active array trenches are narrow, ranging from1 μm down to 0.35 μm (and preferably 0.4 μm) using 0.35-μm class waferfabs. In advanced deep-submicron wafer fabs, structures could further bereduced down to 0.1 μm (so long as it is cost effective). Consideringthe fiscal impact of equipment and facilities depreciation on wafermanufacturing costs, 0.25-μm-class fabs will likely constitute thesuccessor to the 0.35 μm power MOSFET wafer production manufacturing nowramping up. In such fabs, trenches 0.25 μm wide can be employed.

[0079] Additional depth may be added to the gate bus trench tocompensate for the thickness of oxide 435. Furthermore, active trenches433D can be etched to a greater depth (e.g., 0.3 μm) when thick bottomoxide 435 (i.e., 435A through 435E) is included in device fabrication.

[0080]FIGS. 11A to 11Q illustrate an exemplary fabrication process for atrench gated MOSFET with a planarized gate bus in accordance with anembodiment of this invention. In device 450 of FIG. 11A, an N-typeepitaxial layer 453 is grown atop N+ substrate 452. Silicon dioxidelayer 454 is grown by thermal oxidation of epitaxial layer 453 to athickness of 200 A to 1000 A at a temperature of 800 to 1050° C. Usingchemical vapor deposition (CVD), silicon nitride (S_(i3)N₄) 455 isdeposited to a thickness of 600 A to 1500 A. The oxide-nitride sandwichis then patterned by photolithography and an etch process usingphotoresist mask 456 to produce opening 457. After etching siliconnitride layer 455 and underlying oxide 454, wide silicon trench 457 isetched as shown in FIG. 11B using reactive ion etching (RIE) techniqueswell known to those skilled in the art.

[0081] Narrow trench 458 is then photolithographically defined using aphotoresist layer 459 as shown in FIG. 11C to control the etching ofsilicon nitride 455, silicon dioxide 454, and epitaxial layer 453 toform silicon trench 458. After photoresist strip, the resultingstructure shown in FIG. 11D has two trench features: deep and narrow(active transistor array) trench 458 and shallow and wide (gate bus)trench 457. The patterned nitride/oxide sandwich, or hardmask used todefine the two trench types remains at this step. Further fabricationcan be subdivided into two categories: process flows in which thenitride/oxide hardmask is removed, and process flows in which thenitride/oxide hardmask remains during subsequent steps. In theintegrated process flow of Figs. 11A to 11Q, the hardmask persiststhroughout much of device fabrication.

[0082] In FIG. 11E, the exposed silicon trench sidewalls are thenexposed to a sacrificial oxidation at a temperature between 900° C. and1100° C. with 1000° C. being preferred. Sacrificial oxidation 451,typically 100 Å to 700 Å (but preferably 250 Å) is used to removecrystal defects created during the etch process.

[0083] The sacrificial oxide 451 is then removed by etching, commonlywith HF acid. At this juncture, two process options exist. In the “thingate” option, gate oxide is grown uniformly in all narrow and widetrenches and polysilicon is deposited. This thin gate version suffersfrom higher electric fields and increased capacitance compared to thealternate option incorporating thick bottom oxide (TBOX) into theprocess sequence. In the TBOX process flow, a thin lining oxide and athick bottom oxide 471A and 471B are formed before any gate oxide isgrown. As shown in FIG. 11F, the thin lining oxide is grown on the samesilicon surfaces and under similar process conditions to the sacrificialoxide. Lining oxides may range in thickness from 100 Å to 1000 Å. Thickbottom oxide 471 (i.e., 471A and 471B) is then deposited by directionaldeposition using high-pressure CVD. U.S. Pat. No. 6,291,298, entitled“Process of Manufacturing Trench Gate Semiconductor Device Having GateOxide Layer with Multiple Thicknesses” and U.S. Pat. No. 6,413,822,entitled “Super-Self-Aligned Fabrication Process of Trench-Gate DMOSwith Overlying Device Layer” further describe processes for formation ofoxide layers in trenches and are incorporated by reference in theirentirety. The sidewall oxide on the vertical sidewalls (comprising boththe thermally oxidized lining oxide and any sidewall oxide residue) isetched off and gate oxide 470A regrown (also under similar processingconditions to the sacrificial and lining oxides). Final gate oxidethicknesses range from 100 Å to 1300 Å with 500 Å being common for 20 Vgate operation, 300 Å common for 12 V gate operation, and 200 Å commonfor 8 V gate operation. During directional deposition, thick oxide 471Aaccumulates at the bottom of narrow trench 458 and thick oxide 471Baccumulates at the bottom and on the sidewall of wide trench 457. Thevertical sidewall of narrow trench 458 prevents significant oxidedeposition during directional deposition, but in a wide trench, thedeposition is more conformal and graded. The thick oxide 471C and 471Dalso accumulates on the top of the silicon nitride 455 on the siliconmesas between trenches 458 and 457.

[0084] In FIG. 11G, polysilicon 472 is deposited onto device 450,filling narrow trenches 458 and wide gate bus trenches 457. Thepolysilicon is deposited using CVD to a thickness of 0.7 μm to 3 μm solong as the trenches are filled. The polysilicon may be depositedundoped and later implanted (or doped via predeposition from a gaseousor solid dopant source like POCl₃ or BN). Preferably the polysilicon canbe doped in-situ, i.e., as it is being deposited, with phosphorus (forN+ poly) or boron (for P-type poly). Depositing the polysilicon intotrenches of differing shapes, e.g., wide shallow trenches 457 for gatebussing and narrow deeper trenches 458 for active arrays of verticalMOSFETs, offers unique advantages since a single polysilicon depositionproduces both structures simultaneously. Using a single depositionfollowed by a planarization step also insures good co-planarity betweenthe two polysilicon surfaces, avoiding unwanted topography and potentialgate shorts.

[0085] The surface of the device is then planarized using chemicalmechanical polishing (CMP). Unlike etch-back, CMP is highly planar,removing layers parallel to the silicon's surface. In the cross sectionof device 450 in FIG. 11G, the CMP planarization is performed down tothe cut line labeled A-A′, grinding back through polysilicon 472,completely removing thick oxide residues 471C and 471D from atop thesilicon mesas, and finally stopping either at the silicon surface, or(as shown in FIG. 11H), stopping after the silicon nitride hard masklayer 455 becomes exposed and partially removed. The slower etch rate ofthe mechanically harder silicon nitride naturally acts as an etch stop.

[0086] The resulting structure is shown in FIG. 11H. Using CMPplanarization, polysilicon 472 is split into two pieces (in this crosssectional view), one being gate polysilicon 473A inside narrow trench458 (and similarly in all active area trenches across the device), theother being gate bus polysilicon 473B located in wide trench 457. Thegate and gate bus polysilicon pieces 473A and 472B still remainelectrically shorted (despite their appearance in the cross section ofFIG. 11H) because they are physically connected in a differentthree-dimensional cross section than the one shown.

[0087] The planarity of the surface of device 450 in FIG. 11H isextremely difficult to achieve using chemical etch-back techniques sincemost chemical etchants (such as HF+HNO₃+H₂O₂) are generally isotropic,etching all portions of a common material at a single rate. Isotropicetching is especially characteristic of polysilicon etching sincepolysilicon has no single crystalline orientation. Using isotropicetching, thicker areas therefore etch longer and clear (i.e., areremoved from the surface) later than thinner regions, causing unwantedtopography to remain even if an unreasonably thick (and thereforeexpensive) polysilicon layer is deposited prior to etch-back. It ispossible, however, to combine chemical etch-back and CMP steps insequence, e.g., first thinning and planarizing by CMP then (after thesurface is planar) to continue thinning using etch-back the rest of theway. Alternatively using etch-back thinning first, followed by a shorterduration CMP planarization may reduce the process time withoutcompromising planarity.

[0088] After planarization, a short chemical etch-back of polysilicon isneeded to recess the polysilicon below the surface of epitaxial layer453 to accommodate an oxide cap atop the polysilicon (see FIG. 11I).This etch-back may range from 0.1 μm to 0.5 μm, but should not result inthe surface of the gate polysilicon 473A being deeper than the junctiondepth of the transistor's source region. In FIG. 11J, an optionaloxidation of exposed gate polysilicon 473A and gate bus polysilicon 473Bis performed to seal the polysilicon in high quality thermal oxides 480Aand 480B respectively. The top oxide also acts as a diffusion barrieragainst any subsequently deposited glasses that may contain dopants suchas boron or phosphorus. Dopants may be introduced into the silicon mesasat this step of the process (not shown), preferably by ion implantation.Either chained high-energy ion implantations or a shallow implantfollowed a high-temperature diffusion may be used to form the body ofthe MOSFET. The source may also be implanted at this time or at a laterstep.

[0089] In FIG. 11K, a glass or dielectric layer 481 such as TEOS(tellurium doped glass), PSG (phosphosilicate glass), BPSG(borophosphosilicate glass) or SOG (spin-on glass) is deposited and thenplanarized and thinned using CMP, etch-back, or a combination thereof.Planarization and thinning continues until nitride 455 is exposed beyondcut-line B-B′ as shown in FIG. 11L, whereby dielectric layer 481separates into two islands, region 482A sitting atop the embedded gatein narrow trench 457, and region 482B sitting atop the wide trench gatebus 458. Intervening thermal oxides 480A and 480B prevent dopantcontamination into polysilicon 473A and 473B from doped glass 482A and482B.

[0090] In FIG. 11M, a contact mask is used to define an opening inphotoresist 483 open only atop the wide gate bus polysilicon structure458 but not atop the active array of trenches. In FIG. 11N, the contactis subsequently etched using dry etching techniques to expose gate buspolysilicon layer 473B. Oxide etching is preferably performed using dryetch techniques such as reactive ion etching (RIE) or plasma etching,which are well known to those skilled in the art.

[0091] In FIG. 110, silicon nitride 455 is stripped using an etchant ora dry etch chemistry that selectively etches nitride layers faster thanoxide. In this manner, capping oxides 482A and 482B suffer littleerosion from the nitride's removal. This operation clears the siliconmesas except for thin oxide 454. Dopants may be introduced at this steprather than earlier in the process if so desired. In FIG. 11P, a shortthermal cycle (e.g. 10 minutes at 850° C. to 900° C.) is used to flowglass island 482A and 482B (smoothing their shape) followed by a shortoxide etch or HF acid dip to remove thin mesa oxide 454.

[0092]FIG. 11Q illustrates the final cross section of a devicefabricated in accordance with this invention whereby TiN barrier metal490 (including regions 490A, 490B, and 490C) is deposited. Thereafter,thick metal 491 (including regions 491A, 491B, and 491C) typicallycomprising Al—Cu—Si is deposited using sputtering or evaporationmethods. Finally a metal mask is used to define and etch the metal andbarrier metal into source electrodes 491A and 491C and gate buselectrodes 491B. The resulting device 451 has a high cell density, alarge active contact area (extending from trench to trench edge), afully planarized gate bus and virtually no topography.

[0093]FIG. 12 illustrates the three-dimensional construction ofpolysilicon 1000 comprising wide shallow gate bus 1003 andactive-cell-array embedded gates 1004A, 1004B, 1004C, and 1004D of atrench-gated MOSFET with a planarized gate bus made in accordance withan embodiment of this invention. The surrounding silicon has beenremoved to reveal the 3-D structure of the polysilicon 1000. Gate bus1003 includes stripe contact 1005 to provide continuous shorting of thegate bus by its overlying metal strapping (metal not shown). Theparallel array of trench gates and gate busses is connected by thetransverse polysilicon element 1002 which may be constructed using anarrow deep trench (as shown) or as a shallow trench similar to the gatebus 1003B. The surface of transverse bus 1002, wide gate bus 1003, andactive cell array trench gates 1004A through 1004D as shown all share asubstantially planar surface, a characteristically unique structuralaspect of this invention.

[0094]FIG. 12 also illustrates how the embedded gates abut transversebus 102 as a T-shape. In conventional trench MOSFETs using etch-backmethods, the “T” shaped comer suffers from a depression or dimple in thetop of the polysilicon due to the larger dimension of the trench opening(on the diagonal), making etch-back difficult to control. Using CMPplanarization, the over-etching at the “T” shaped polysilicon junctionis not a problem. Wide gate bus 1003 abuts transverse polysilicon bus102 illustrating that the wide and the narrow trenches do in fact abut.Alternate layouts may also have narrow and wide trenches cross oneanother forming a “+” shaped junction, which would be even moredifficult to manufacture using etch-back processes.

[0095]FIG. 13A illustrates device 1050 fabricated in steps similar tothat of the device 450 of FIG. 11K, except that the planarization of thetop dielectric or glass 1062 utilizes a CMP planarization down to cutline C-C′, essentially down to the silicon surface itself. In this flow,the material located above the silicon surface is removed by the CMPplanarization step, including the deposited glass 1062, nitride 1061,and all or part of thin mesa oxide 1060. The resulting structure isshown in FIG. 13B where only the portion of the deposited glass 1062remaining after planarization are cap oxides 1063A and 1063B over thenarrow and wide trenches, respectively. The structure of FIG. 13B can becontacted by masking and etching a contact window for the gate buspolysilicon 1056B. Alternatively, an additional planarization step canbe added by depositing glass, silicon nitride, or other dielectric 1068,then performing an optional CMP or planarizing etch-back as shown inFIG. 13C. A contact window can then be defined photolithographicallyusing photoresist 1061 as shown in FIG. 13D, exposing only the gate busregion (or optionally exposing portions of active areas but not exposingthe top of the trenches in the active array). Etching the gate buscontact window is next performed using a plasma etch, reactive ion etch,or even a wet chemical to sequentially remove dielectric 1068, glasslayer 1063B, oxide layer 1058, finally stopping on gate-bus polysiliconlayer 1056B. After stripping photoresist 1061, the resulting structureis shown in FIG. 13E. Next, dielectric 1068 is stripped or removed usinga plasma etch process as shown in FIG. 13F, preferably using a selectiveetch that etches dielectric layer 1068 at a much faster than glasslayers 1063A and 1063B. In this way, the final etch used to expose theactive area mesa regions does not erode or damage the dielectric cap1063A located atop the active array trench.

[0096] Thereafter, a barrier metal 1070A and 1070B such as TiN isformed, followed by a deposition of a thick metal 1071A and 1071B suchas aluminum-silicon, Al—Si—Cu, or copper, and mask and etch process forthe metal then completes fabrication of trench-gated MOSFET 1050 asshown in FIG. 13G.

[0097] To enhance the dielectric thickness atop the trench gatesfurther, MOSFET 1056 of FIG. 13D can be masked and etched usingpatterned photoresist 1061 with contact windows open to the siliconmesas and the gate bus but not over the trench gates themselves. Such adevice comprises a non-self-aligned contact mask version of a verticaltrench-gated power MOSFET with a planarized gate bus.

[0098]FIGS. 14A to 14P illustrate an alternate fabrication sequence fora trench-gated vertical power MOSFET 1100 having a planarized gate busin accordance with an embodiment of the invention.

[0099] In FIG. 14A, mask 1103 is applied and patterned on epitaxiallayer 1102, grown atop substrate 1101. Photomasking and optional etchingtechniques open a wide window and then a shallow trench to form gate-bustrench region 1105. Optionally, an implanted layer of dopant (not shown)may be introduced into the etched trench area while mask 1103 is stillin place. In such an event, the energy of such an implant must notpenetrate the blocking mask 1103 except in the wide trench etched area.

[0100] In FIG. 14B, a second mask 1104 is applied and patterned to formnarrow opening 1106. Mask 1104 must step down into trench 11 05 withoutcracking or breaking while crossing the step formed at the edge oftrench 1105. This goal may be accomplished by slightly edge-rounding thetrench etch used to form gate bus trench 1105, using thick photoresist,having a thickness greater than 1 μm, or using a conformal dielectric,either thermally grown oxide or a conformally deposited material such assilicon nitride to form mask 1104. After the narrow trench 1106 isetched, mask 1104 is removed as shown in FIG. 14C. The silicon may thenbe oxidized, and etched to remove defects using any sacrificialoxidation process, well known to those skilled in the art. Thereafter,gate oxide 1110 is grown uniformly across the silicon surface and alongthe etched sidewalls and bottom portions of gate bus trench 1105 andactive trench 1106 as shown in FIG. 14D. Polysilicon 1111 is thendeposited with optional in-situ doping in FIG. 14E, followed by aplanarization using CMP with or without a planarizing etch-back step.

[0101] During the planarization step, the exposed surface becomesseparated into three regions including wide planarized gate bus region1105, active-array narrow trenches 1106, and intervening silicon mesas.The resulting structure is shown in FIG. 14F. During the planarizationprocess, polysilicon 1111 and gate oxide 1110 are split along theillustrated cross-section into active-array embedded gate polysilicon1113A and gate oxide 112A (located within narrow trench 1116); and gatebus polysilicon 1113B and lining oxide 112B in wide trench 1105.Ideally, gate bus lining oxide 1112B should be thicker than gate oxide1112A to reduce parasitic capacitance, but this feature requires extraprocessing steps not shown in this sequence. Oxidizing the wide trenchprior to forming the narrow gate trench is, however, compatible with theprocess to form device 1100.

[0102] In FIG. 14G, the gate and gate-bus polysilicon regions,respectively 1113A and 1113B are over-etched into the respectivetrenches to facilitate embedding the trench gates below the surface. InFIG. 14H, a thin oxide 1115B, typically 100 Å to 700 Å is grown bythermal oxidation. During this operation, the embedded polysilicon gates1113A and gate bus 1113B become partially consumed to grow oxide 1115Aand 1115B. Likewise, unprotected by silicon nitride, the silicon mesaportions of epitaxial layer 1102 also become oxidized. Next, glass 1116is deposited, as shown in FIG. 14I and subsequently planarized using CMPor etch-back methods to produce the cross section of FIG. 14J. Unlike inother illustrated embodiments in accordance with this invention, in FIG.14J, the glass layer 1116 is not etched all the way back to the surfaceof the silicon mesas.

[0103] In FIG. 14K, photolithographically patterned photoresist 1117forms a contact mask protecting regions of glass layer 1116 over theactive area trenches 1106 and the edge of the wide trench gate busregion 1105. The creation of contact mask 1117, however, requiresalignment to the trenches, thereby limiting the maximum density oftrenches in the device. In FIG. 14L, the glass layer 1116 is etched andoptionally flowed, i.e., heated slightly (e.g., at 900° C. for 10minutes) so the glass redistributes to produce smooth edges, especiallyatop trenches 1106 where metal voiding between the trenches is apotential risk (as described in the background of this disclosure). Thesmooth edges of post-flow glass 1116 covering the gate bus 1105 edgealso prevent metal breakage and step coverage issues.

[0104] Lastly, barrier metal 1120A and 1120B (such as TiN), is thendeposited, followed by thick metal 1121A and 1121B (such as Al—Cu—Si),and subsequently patterned into source metal and gate bus metal regionsas shown in FIG. 14M.

[0105]FIGS. 14N to 14P illustrate an alternate process flow thatinvolves performing the contact opening using a two-step, two-maskprocess to etch glass 1116 when forming a device 1200. Starting with afirst contact mask and photoresist 1125, this first contact mask permitsetching glass 1116 only atop the gate bus region 1105 while contact mask1125 protects the remainder of the device from etching as shown in FIG.14N. A second contact mask is then defined in photoresist layer in FIG.14O that protects the gate bus region and exposes only the active arrayof trenches to etching. After contact etching, metallization isperformed, followed by patterning and etching, resulting in the deviceshown in FIG. 14P, where glass 1116 covers the silicon surface betweenmetal regions 1121A and 1121B.

[0106]FIGS. 15A to 15I illustrate the fabrication steps of an alternateprocess for forming a trench-gated MOSFET with a planarized gate bus,employing a thick bottom oxide process to form thick oxides at thebottom of narrow active trenches and a thick oxide lining of the widetrench gate bus structure.

[0107] In FIG. 15A, device 1250 includes a silicon substrate 1251 andepitaxial layer 1252 in which already completed fabrication steps haveformed a wide trench and a narrow trench in accordance with thisinvention. Subsequently the surface of the mesas and trenches isthermally oxidized to form lining oxide 1253, preferably to a thicknessof 200 Å to 1000 Å. Thereafter, thick bottom oxide 1254F is depositedusing directional deposition in accordance with U.S. Pat. No. 6,291,298,which was incorporated by reference herein above. The directionallydeposited thick bottom oxide, or TBOX, is formed through high-densityplasma CVD to a thickness of 1000 Å to 4000 Å at the trench bottoms.During the TBOX deposition some oxide 1254E forms on the narrow verticaltrench sidewall, which may easily be removed by a short acid dip indilute HF acid. To minimize the sidewall deposition during TBOXprocessing, the narrow trench's sidewall must form an angle (relative tothe mesa surface) greater than 90% and ideally over 94%, which alsofacilitates planarizing refills. The TBOX process also produces thickoxide regions 1254A and 1354B atop the silicon mesas and 1254D in thebottom of the wide trench. Also shown in FIG. 15A, oxide 1254C isdeposited along wide trench sidewall, which is significantly thickerthan vertical sidewall oxide 1254E. The angle of the wide trench'ssidewall must be greater (more-sloped) than that of the narrow trenches,and ideally should form an angle of around 30° to 50°.

[0108] The thick sidewall oxide 1254C along with the TBOX bottom oxide1254D formed in the wide gate bus trench benefits device 1250 byreducing electric fields and substantially lowering gate-to-sourcecapacitance associated with the gate bus structure. The capacitance of aplanarized gate bus having a 3000 Å oxide formed using the TBOX processis one-fifth to one-tenth that of a device employing only a gate oxideto line the wide trench.

[0109] After the short acid dip (to remove the vertical sidewall oxidein the narrow trench), gate oxide 1255 is thermally grown as shown inFIG. 15B. Since thick oxide 1254 covers the rest of the device, theamount of oxidation in the wide trench and atop the silicon mesas is notsubstantial. The two trenches are then simultaneously filled with asingle polysilicon CVD operation as shown in FIG. 15C. Beyond this step,the process flow is similar to the other flows previously described inthis disclosure, including planarizing etch-back or CMP shown in FIG.15D, polysilicon over-etch followed by thermal oxidation 1262A and1262B, and dielectric or glass deposition 1263 in FIG. 15E, secondplanarizing CMP or etch-back in FIG. 15F, followed by any contact masksequence described in accordance with this invention. In FIG. 15G, forexample, a contact mask is performed to contact trench gate bus 1261B,followed by resist stripping and cleaning to clear the mesa regionsshown in FIG. 15H, and finally by metallization and metal etching inFIG. 15I.

[0110] In the exemplary devices and process described thus far, noprovision has been included to reduce the signal propagation delaythrough the array of active trench gates (aside from heavily doping thepolysilicon), nor to eliminate the need for strapping the thick metalalong the length of the gate bus. FIGS. 16A to 16H, however, illustratea modification to the aforementioned devices and processes to form atrench-gated MOSFET with a planarized gate bus in accordance with anembodiment of the invention that introduces a high-conductivity metalinto the trench devices and into the gate bus, thereby enabling higherspeed or higher frequency device operation.

[0111] Starting with device 1300 shown in FIG. 16A, previously describedprocesses form narrow and wide trench regions 1304 and 1305 respectivelyin an epitaxial layer 1302 atop substrate 1301. Thick bottom oxide 1301Dhas been formed in narrow trench 1304, and trench 1305 contains thickoxide 1310C (including a thick sidewall oxide). Gate oxide 1311 linesthe vertical sidewall of narrow trench 1304.

[0112] In FIG. 16B, a thin polysilicon layer 1312, from 200 Å to 2000 Åbut preferably around 400 Å to 900 Å is deposited. Since the depositionproduces a thin layer, polysilicon 1312 is relatively conformal andshould not fill or overfill narrow trench 1304. This step issignificantly different from prior trench process described herein or inthe industry since the polysilicon intentionally should and must notfill the narrow trench 1304.

[0113] In FIG. 16C, a high-temperature refractory metal 1313 such astungsten, platinum, titanium, chromium, or cobalt is deposited bysputtering, chemical vapor deposition, or chemical decomposition usingmethods well known to those skilled in the art. Evaporation is not apreferred means of deposition for high temperature metal since theirmelting point can be well over 1100° C. The deposited metal may be 100 Åto 2000 Å in thickness but is preferably in the range of 500 Å to 1500Å. The thickness of the deposited metal should be chosen such that twicethe combined thickness of gate oxide 1311, sidewall polysilicon 1312,and high-temperature metal 1313, should not significantly exceed thewidth of narrow trench 1305. In other words, significant overgrowth ofmetal 1313 outside and over the top of narrow trench 1305 is notdesirable since it can make subsequent processing and planarizationdifficult and prone to gate shorts.

[0114] A short acid dip or etch is performed in FIG. 16D to clear anymetal or metal residue from the trench sidewalls, especially in thenarrow trench active array regions. High temperature metal 1313 isthereby divided into pieces 1314A and 1314B on top of the silicon mesasand into pieces 1314D and 1314C located at the bottom of the trenches.

[0115] In FIG. 16E, a second polysilicon 1315 is deposited to asubstantial thickness, ideally over 4000 Å, to fill and overflow thetrenches and in so doing, planarize the structure. Moreover, secondpolysilicon layer 1315 seals the high temperature metal 1314D inside alining of polysilicon comprising polysilicon layer 1312 underneath and1315 on its top, all contained within narrow active array trenches.Polysilicon 1315 also seals high temperature metal 1315C inside the widegate-bus trench, surrounded by first polysilicon 1312 underneath andsecond polysilicon 1315 atop high temperature metal 1314D. On thesilicon surface (i.e., atop the mesa regions), second polysilicon 1315covers high temperature metal regions 1314A and 1314B.

[0116] Next the wafer is planarized, preferably with CMP, to remove theoverlying polysilicon, high temperature metal, and dielectrics lyingabove the mesa silicon surface. The result is shown in FIG. 16F,illustrating the complete removal of the surface layers, now separatedand contained entirely within narrow trench 1304 and wide gate-bustrench 1305. Specifically, narrow trench 1304 now contains the followingelements: thick bottom oxide 1310D, gate oxide 1311, polysilicon 1316A(comprising both first and second polysilicon depositions), and embeddedhigh temperature metal or silicide 1314D. Wide trench 1305 is filledwith thick lining oxide 1319C, polysilicon 1316B, and embedded, hightemperature metal or silicide 1314C.

[0117] At this step, dopant (not shown) may easily be introduced intothe silicon mesa regions to form the DMOS body and source regions,either by chained ion implantation or by ion implantation and subsequentdiffusion. In FIG. 16G, the top portions of polysilicon regions 1316Aand 1316B are etched back, oxidized to form thin oxide 1321A and 1321B.Dielectric deposition (e.g., using glass, silicon nitride, or anotherdielectric) and etch-back or CMP then form top dielectrics 1320A and1320B. In FIG. 16H, etching of a contact opening for the gate bus andthen metallization is completed to form barrier metal 1327A and 1327B,and thick metal 1329A (source) and 1329B (gate bus).

[0118] Regions may also be formed containing wide trench 1305 where thegate contact and gate metal 1327B are excluded. Such regions may in factbe covered with source metal 1327A yet still serve to propagate gatesignals quickly along its embedded gate metal 1314C.

[0119] In every process shown with high temperature metal, the metal mayreceive a heat treatment, such as 30 minutes at 950° C. to react themetal and the polysilicon layers to form a metal-silicide such asplatinum-silicide, tungsten-silicide, etc. Without annealing, the metaland polysilicon layers remain separate and distinct, and theintermetalic combination phase of metal-rich silicon is not formed attheir interface.

[0120]FIG. 17 illustrates the polysilicon portion of the device in 3Dprojection made in accordance with this invention with the silicon anddielectrics portions removed. The device contains active-arraypolysilicon gate portions 1354A, 1354B, 1354C, and 1354D with embeddedhigh temperature metal or silicide portions 1355A, 1355B, 1355C and1355D, respectively. A polysilicon gate bus contains parallel portions1351 and transverse portions 1352 with embedded high temperature metalor silicide 1355E and 1355F respectively. Gate bus also includesoptional contact area 1353 running along the top of the gate bus regionsand shorted by top metal (not shown). Such a polysilicon gatefacilitates rapid propagation of signals across a large area device,while offering the benefit of co-planarity with the top of gate bus andactive polysilicon regions.

[0121] In another aspect of this invention, dopant atoms may beintroduced into the portion of silicon below the wide trench gate bus.In a preferred embodiment 1370 shown in FIG. 18A, dopant is introducedinto wide trench 1374 by ion implantation to form doped region 1380,whose depth may be determined by ion implantation energy, or by anoptional subsequent diffusion. The implant may comprise a single implantor chain implant, and is generally preferable to be of the oppositeconductivity type to epitaxial layer 1372. If for example, epitaxiallayer 1372 is N-type, then region 1380 may be P-type, doped with boronto a concentration ranging from slightly higher than the epitaxialdoping to a concentration as high as 5E19 cm⁻³ but preferably in therange of 1e7 cm⁻³ to 4E18 cm⁻³.

[0122] The depth of doped region 1380 may extend from 0.1 μm to severalμm, but generally should not extend substantially deeper than the depthof a narrow trench gate 1375 as shown in FIG. 18B. Ideally, if thickbottom oxide is employed, the depth of region 1380 should extend deeperthan the polysilicon gate, but more shallow than the narrow active-arraytrench 1375 itself. For example, if the active narrow trench 1375 is 1.6μm deep containing a thick bottom 1382 oxide 3000 Å thick, then thedepth of region 1380 may be targeted for 1.4 μm relative to the silicon(mesa) surface, deeper than the polysilicon gate but shallower than thenarrow trench 1375. If the wide trench 1374 is etched to a depth of 0.7μm, the implant or diffusion of region 1380 would result in a depth 0.7μm below the bottom of the wide trench 1374 after etching.

[0123] Implants may range from 50 keV to 2 MeV but the deepest implantpreferably will have maximum implant energy of 400 keV. Implant dosesmay range from 2E12 to 3E15 cm⁻² but typically will be in the range of5E13 to 7E14 cm⁻². After the gate and gate bus trenches are formed inaccordance with this invention, the DMOS body regions 1383A and 1383Bmay be introduced by ion implantation and diffusion, or by chained ionimplantation as described previously. Since the body region and the PWregion are of the same conductivity type, the two P-type regions mergeto form a complete enclosure of dopant surrounding wide trench 1374,shielding the gate bus from high drain potentials.

[0124] In FIG. 18C, shallow N+ source regions are introduced inaccordance with this invention, having concentrations in the range of4E19 to 7E19 cm⁻³ and a depth of 0.2 μm to 0.5 μm. Implant doses mayrange from 3E15 cm⁻² to 7E15 cm⁻² and implant energies may range from 20to 80 keV for phosphorus or 60 to 139 keV for arsenic. N+ source 1394Aand 1394B may be masked by photoresist 1395 but otherwise self align totrench gates 1375, just as P-type body 1383A and 1383B does. The channellength of the vertical DMOS device is set by the difference of theP-type body 1383 junction depth and the N+ source 1394 junction depth.Shallow P+ regions 1397 are then introduced to improve contact to theP-type body regions 1383, as shown in FIG. 18D. The shallow implants maybe masked by photoresist 1396 or performed as a blanket implant. Boronimplantation to form P+ 1397 may be in the range of 7E14 cm⁻² to 7E15cm⁻² but should not substantially counter-dope N+ 1394, especially inthe case where a blanket P+ implant is used. Implant energies may rangefrom 30 keV to 120 keV but should not result in a P+ depth substantiallydeeper than N+ 1394 in the event of a blanket P+ implant, otherwise thechannel concentration of P-type body region 1383 along the sidewall ofactive trench 1375 may be disturbed.

[0125]FIGS. 18A to 18D are not meant to be limiting in process sequence,but to exemplify that dopants may be introduced into the structuresformed in accordance with this invention (combining narrow activetrenches and wide shallow trenches) to produce trench-gated verticalDMOS devices.

[0126]FIGS. 19A to 19D illustrate the cross sections of an alternateprocess to form a trench-gated MOSFET with a planarized gate bus. Inthis example, the wide trench is not formed by trench etching but byLOCOS oxidation and oxide removal. FIG. 19A starts with a patternedoxide (1404) nitride (1405) hard mask similar to prior descriptionsherein. Next, wide gate bus region 1403 is exposed to oxidation,typically to form an oxide 1406 having a thickness of 3000 Å to 7000 Åas shown in FIG. 19B. A portion of oxide 1406 lifts silicon nitridelayer 1405 to form the familiar “birds beak” shape common to LOCOS fieldoxidation processes. The field oxidation process may be performed from800° C. to 1100° C. but typically at 950° C. for a time of 30 min toseveral hours. The encroachment of oxide under the silicon nitride layer1405 can cause stress, which is relieved by oxide 1404, already usedpreviously as part of the trench hard mask.

[0127] In FIG. 19C, narrow trench 1407 is formed by masking and etchingwhile oxide 1406 remains protected. Alternatively, if an etch chemistryof the plasma etching has good selectivity between silicon and oxide,then layer 1406 can remain exposed during the silicon trench etch. Evenif it is partially removed, it does not affect subsequent devicefabrication so long as the dry etch does not penetrate through oxide1406 (an unlikely event). In FIG. 19D the silicon nitride and all oxideregions are stripped, leaving silicon epitaxial layer 1402 containing awide sloped trench region 1403 (formed by oxidation) and narrow activetrenches 1407. The depth of wide gate bus trench 1403 is roughly halfthat of the oxide thickness grown. Accordingly, this method is notpractical for wide trench regions deeper than approximately 0.4 μm,since the very thick oxidation would require excessive time and producetoo much stress at the nitride edge.

[0128] After wide and narrow trenches have been formed, fabrication of atrench gated DMOS with a planarized gate bus may proceed usingpreviously described methods in accordance with this invention.

[0129] An alternate process used to form a trench gate MOSFET 1450 inaccordance with this invention is shown in FIGS. 20A to 20E. Startingwith a hard mask in FIG. 20A, followed by a thick oxidation in FIG. 20B,an oxide etch is next performed to remove field oxide 1456 while leavingnitride 1454 and oxide 1455 hard-mask in place. As shown in FIG. 20C,some nitride overhang may result, which can be dipped off in a shortacid etch. In FIG. 20D, gate bus boron implant 1458 is next introducedinto wide trench 1455, while the hard mask layers 1453 and 1454 andoptional photoresist 1457 layers block the implant from penetrating themesa regions. In FIG. 20E, narrow trench 1460 is defined and etched,leaving portions of nitride 1454 and oxide 1453 intact. Fabrication thenmay proceed in accordance with this invention.

[0130] Another alternate process used to form a trench gate MOSFET 1470in accordance with this invention is illustrated in FIGS. 21A to 211.After wide trench 1475 shown in FIG. 21A has been formed by eithertrench etch or LOCOS methods, and subsequently implanted to form dopedregion 1476, then wide trench 1475 is oxidized to form lining oxide 1477as shown in FIG. 21B. Lining oxide may be 1000 Å to 3000 Å in thickness.In FIG. 21C, polysilicon 1478 is deposited and planarized by etch-backor CMP as shown in FIG. 21D, where the polysilicon is contained entirelyin wide trench 1475. Thereafter, in FIG. 21E, narrow trench 1480 isetched, followed by the formation of gate oxide 1485, second polysilicon1481, high temperature metal 1482, and lastly third polysilicon 1483 inFIG. 21F.

[0131] Planarization of the wafer by CMP results in the structure shownin FIG. 21G, having different polysilicon in the gate bus 1475 andnarrow gate trenches 1480, where only the narrow gates contain the hightemperature metal or silicide 1482. In FIG. 21H, the polysilicon isetched back into the trenches and a capping dielectric or glass isdeposited and planarized to form dielectric caps 1491A on the activetrench 1480 and 1491B on the gate bus trench 1475, as shown in FIG. 21I.

[0132] Alternatively, in device 1500 of FIG. 22A, wide gate bus trenchcan be oxidized to form lining oxide 1504, followed by a firstpolysilicon deposition 1505, and a high temperature metal deposition1510. Metal 1510 is then dipped in acid to remove metal from the trenchsidewall, leaving metal 1511B at the bottom of the trench and 1511A onthe top of the structure as shown in FIG. 22B. Next, a secondpolysilicon is deposited in FIG. 22C, followed by CMP planarization toform a planarized gate bus containing embedded high temperature metal orsilicide 1511B encased by polysilicon 1505 and 1520 shown in FIG. 22D.

[0133] In FIG. 22E, narrow trenches 1519 are next formed, followed bythick bottom oxide 1522 formation in FIG. 22F, a third polysilicon 1525deposition and high temperature metal 1526 deposition in FIG. 22G, and afourth polysilicon in FIG. 22H. Planarization using CMP next occurs asshown in FIG. 22I, followed by polysilicon etch-back and dielectriccapping and planarization in FIG. 22J. Silicon nitride 1507 is removedto facilitate mesa contacts in FIG. 22K. Fabrication is completed by amasked contact etch to contact the wide gate bus along with barriermetal and metal deposition and masking to produce the device shown inFIG. 22L. While complex in its fabrication and relatively expensive tomake compared to the other devices described in accordance with thisinvention, device 1500 of FIG. 22L allows completely independent controland adjustment of the gate bus and active gate structures.

[0134]FIGS. 23A to 23H illustrate some variants of a trench gate withembedded high temperature metal or silicide. In FIG. 23A, device 1600includes trench 1605 with uniform gate oxide 1606, embedded metal 1608completely enclosed by polysilicon layers 1607 and 1609. This approachprevents any risk of metal contamination to gate oxide quality butrequires two polysilicon depositions. In FIG. 23B, device 1620 includesa trench gate with a thick bottom oxide 1625, gate oxide 1620, andembedded metal 1628 completely enclosed by polysilicon layers 1627 and1629. This approach also prevents any risk of metal contamination togate oxide quality and requires two polysilicon depositions as well as athick bottom oxide deposition.

[0135] In FIG. 23C, device 1640 includes trench gate with uniform gateoxide 1645, embedded metal 1647 enclosed below by polysilicon layer 1646but only sealed by glass 1648 on top. This approach reduces any risk ofmetal contamination to gate oxide quality (because of the polysiliconlining) but does not eliminate it. Device 1660 in FIG. 23D is the thickbottom oxide version of device 1640 with thick bottom oxide 1665, liningpolysilicon 1666 and embedded metal 1667, but only a dielectric cap 1668to seal the metal. This approach reduces the risk of metal contaminationto gate oxide quality (because of the polysilicon lining) but does noteliminate it.

[0136] In FIG. 23E, device 1680 allows a high temperature metal orsilicide to come in direct contact with the gate oxide 1685. Using thisstructure requires lower temperature processing in all subsequent stepsto prevent metal migration into the gate oxide. The embedded metal 1688is located at the bottom of the trench and capped by polysilicon layer1686. The metal 1688 should not overlap the p-type body region 1683 orthe threshold of the device may vary significantly (since metal has adifferent work function than doped polysilicon does).

[0137] In FIG. 23F, device 1700 also allows a high temperature metal orsilicide to come in direct contact with the gate oxide 1709 but not withthe bottom of the trench (since thick bottom oxide 1705 protects thetrench bottom). Using this structure requires lower temperatureprocessing in all subsequent steps to prevent metal migration into thegate oxide. The embedded metal 1706 is located below the body and on topof the thick bottom oxide 1705 and is capped by polysilicon layer 1707.The metal 1706 should not overlap the p-type body region 1703 or thethreshold of the device may vary significantly (since metal has adifferent work function than doped polysilicon does).

[0138] In FIG. 23G, device 1720 also allows a high temperature metal orsilicide to come in direct contact with the gate oxide 1725, but only atthe top of the trench. Using this structure requires lower temperatureprocessing in all subsequent steps to prevent metal migration into thegate oxide. The embedded metal 1728 is located above the body 1723 andon top of the polysilicon gate 1727. Only dielectric layer 1729 caps themetal 1728. The metal 1728 should not overlap the p-type body region1723 or the threshold of the device may vary significantly (since metalhas a different work function than doped polysilicon does). Device 1740in FIG. 23H is similar to device 1720 except that the trench includesthick bottom oxide 1745.

[0139]FIG. 24 illustrates some various process flow sequences used tofabricate devices shown in FIGS. 11A to 11Q, FIGS. 13A to 13G, FIGS. 14Ato 14P and FIGS. 15A to 15I, and possible variants thereof. Thesedevices do not include silicided gates. FIG. 25 illustrates the modifiedprocess flows needed to embed metal into the trench gate and gate busstructures. FIG. 26 illustrates the process flow variations used to formthe shallow wide gate bus trench using LOCOS field oxidation techniques.FIG. 27 illustrates process flows for fabrication of devices havingdamascene gate structures including metal or silicide as illustrated inFIGS. 22A to 22L and 23A to 23H.

[0140] The processes and devices described herein are not limited toN-channel devices. To those skilled in the art, it is well known thatany vertical MOSFET N-channel process can be converted to P-channel byswapping P-type and N-type dopants, and adjusting for implant rangestatistics with implant energies. The structures and methods of forminga trench gate MOSFET with a planarized gate bus are therefore applicableto both N-channel and P-channel devices.

What is claimed is:
 1. A power MOSFET comprising: a substrate comprisinga first trench and a second trench extending from a top surface of thesubstrate; a source region, a channel region, and a drain region in thesubstrate and arranged vertically along at least a portion of a wall ofthe first trench; a gate structure that extends continuously in thefirst and second trenches, wherein a top surface of the gate structuredoes not extend above the top surface of the substrate, a first portionof the gate structure acts as a gate of a vertical device in the wall ofthe first trench, and a second portion of the gate structure resides inthe second trench; and a gate contact that contacts the second portionof the gate structure.
 2. The power MOSFET of claim 1, wherein thesecond trench is wider than the first trench.
 3. The power MOSFET ofclaim 1, wherein the first trench is deeper that the second trench. 4.The power MOSFET of claim 1, further comprising a dielectric materialresiding in the first and second trenches above the gate structure,wherein a top surface of the dielectric material is coplanar with thetop surface of the substrate.
 5. The power MOSFET of claim 1, furthercomprising a metal layer that is patterned to form a source contact thatoverlies the first trench and contacts the source region adjacent to thefirst trench and the gate contact that contacts the gate structure inthe second trench.
 6. The power MOSFET of claim 1, wherein the gatestructure comprises a continuous polysilicon region that extends fromthe first trench to the second trench.
 7. The power MOSFET of claim 6,wherein the gate structure further comprises a metal/silicide region inthe first trench.
 8. The power MOSFET of claim 7, wherein thepolysilicon region in the surrounds the metal/silicide region andprevents direct contact between the metal/silicide and a gate oxideregion.
 9. The power MOSFET of claim 6, wherein the gate structurefurther comprises a metal/silicide region in the second trench.
 10. Thepower MOSFET of claim 9, wherein the metal/silicide region extendscontinuously from the second trench into the first trench.
 11. Afabrication process for a power MOSFET comprising: forming a firsttrench in a substrate; forming a second trench in the substrate; forminga conductive gate structure that extends continuously from the firsttrench into the second trench; doping a mesa in the substrate adjacentthe first trench to form a vertical device including a source region, achannel region, and a drain region that are vertically aligned along awall of the first trench; performing a chemical mechanical polishing(CMP) process on a structure including the conductive gate structure inthe first and second trenches; and depositing a contact layer after theCMP process, wherein the contact layer includes a first regioncontacting the device adjacent to the first trench and a second regionoverlying the second trench and contacting the conductive gatestructure.
 12. The fabrication process of claim 11, further comprisingforming an insulating layer overlying the conductive gate structure. 13.The fabrication process of claim 12, wherein the CMP process removes theinsulating layer from above the mesa.
 14. The fabrication process ofclaim 14, further comprising performing a contact etch that etches anopening through the insulating layer to expose a portion of theconductive gate structure in the second trench, the contact layerfilling the opening and contacting the exposed portion of the gatestructure in the second trench.
 15. The fabrication process of claim 14,performing the contact etch forms a second opening through theinsulating layer that exposes the mesa, the contact layer filling thesecond opening and contacting the vertical device.
 16. The fabricationprocess of claim 12, further comprising: forming a hardmask havingopenings that define boundaries of the first and second trenches,wherein the CMP process exposes a region of the hardmask overlying themesa and leaves the regions of the insulating layer in the openings inthe hardmask; and removing the hardmask without removing the regions ofthe insulating layer left after the CMP process, wherein the contactlayer contacts the mesa where the region of the hardmask was removed andthe regions of the insulating layer separate the gate structure in thefirst trench from the contact layer.
 17. The fabrication process ofclaim 11, further comprising forming a layer of a hardmask material onthe substrate, wherein forming the second trench comprises patterningthe hardmask material to form an opening over an area of the substratecorresponding to the second trench.
 18. The fabrication process of claim17, wherein forming the second trench comprises: performing a LOCOSoperation to oxidize the substrate in the area of the opening in thehardmask material; and removing an oxide region formed during the LOCOSoperation to leave the second trench in the substrate.
 19. Thefabrication process of claim 17, further comprising etching thesubstrate exposed through the opening in the hardmask material tothereby form the second trench.
 20. The fabrication process of claim 15,wherein the second trench is wider than the first trench.